Diesel and gasoline engines emit soot particles, very fine particles of carbon and soluble organics as well as typical harmful engine exhaust gases (i.e., HC, CO and NOx). Regulations have been enacted curbing the amount of soot permitted to be emitted. To meet these challenges, soot filters have been used. The filters must be periodically regenerated by burning off the soot, which results in stresses from axial and radial temperature gradients that can cause cracking of the filter due to stresses caused by the differential temperatures along with the coefficient of thermal expansion of the filter material.
To overcome stresses, ceramic honeycombs, such as catalytic converters, heat exchangers and filters, smaller honeycomb segments are assembled into arrays of segments to form larger honeycomb structure (segmented substrates). Cement layers between the honeycombs have been used, for example, to increase the thermal conductivity to reduce the ultimate temperature reached in the assembled honeycomb such as described by U.S. Pat. No. 6,669,751, incorporated herein by reference. To achieve the improved thermal conductivity, these cements/sealing layers/adhesives have used ceramic particulates to increase the thermal mass/conductivity and ease of application to the smaller honeycomb segments. Often such cements are augmented by the use of ceramic fibers, and ceramic binders and organic binders such as described by U.S. Pat. No. 5,914,187, incorporated herein by reference, to facilitates application of the cement prior to firing (e.g., reduce segregation of particulates) and improve some mechanical properties such as toughness of the cement.
The honeycomb segments that are assembled to prepare these filters do not have perfectly straight surfaces and are not completely flat. When the surfaces bonded together have too much variation of straightness or flatness along the surface, the cement used to bond the surfaces of the honeycomb segments together needs to be thicker than when the surfaces are relatively flat and straight. Thick layers of cement can have deleterious effects on the assembled honeycombs, for instance the backpressure is increased and the thermal stability is decreased. It is known to measure the flatness of segment surfaces see U.S. Pat. No. 6,596,666 and U.S. Pat. No. 7,879,428, incorporated herein by reference, which cite JISB0621-1984 as a test method for measuring flatness. Flatness is generally measured by defining two parallel planes. One plane is defined by the innermost surface of a face of a honeycomb segment, toward the center of the honeycomb segment (least square fit plane of measured points) and the second plane is defined by the outermost surface of the same face of a honeycomb segment. The distance, computed as the difference between outer minus inner, between the planes is known as the flatness and is by definition always positive. Lower flatness numbers are considered better. As a practical matter the surface is mapped by taking several data points (e.g., y and z) and a least square fit plane is calculated mathematically based on the population of points. In production, finished segments are measured for flatness and if a segment has a side which has a flatness which is above the acceptable limit the segment is rejected or scrapped. The scrapping of a significant number of segments adds undesirable costs.
Processes for the preparation of ceramic bodies can result in a number of parts having along a line or a surface, a curved profile (bow). This curved profile may present problems with the use of the ceramic body in the intended use. Where the ceramic body is used to prepare larger ceramic arrays, such curved profiles (i.e. not straight or not flat) may result in the part not being suitable for assembly into a larger array or require too much cement to properly bond the part to other parts.
What is needed is a process for preparing extruded ceramic bodies without a significant number of units that have unacceptable bow. What is needed is a method of preparing segmented ceramic parts having improved flow (e.g. lower back pressure); improved thermal shock resistance and which is more efficient than processes known in the art (e.g. which has a higher rate of segment utilization or lower rate of segment rejection). What is needed is a method of identifying segments that have unacceptable bow or flatness and of repairing the bow or flatness to thereby reduce the scrap rate of production and to enhance the properties of the ceramic bodies and assemblies of ceramic bodies.